The present invention relates to an adjustable sprocket wheel and transmission mechanism for optimizing the energy exerted by the bicyclist, comprising two toothed wheel segments each having a sector angle of substantially 180.degree., which are interconnected by means of lever systems provided with springs arranged in such a way that the sprocket wheel diameter can be changed as bicyclist supplies energy to the cranks which drive the sprocket wheel.
Such a sprocket wheel and transmission mechanism is known from the U.S. Pat. No. 3,798,989. This known sprocket wheel comprises additional small toothed wheel segments which bridge the distance between the 180.degree. toothed wheel segments when the 180.degree. segments are adjusted relative to each other. The linking between the toothed wheel segments and the small toothed wheel segments is such that upon pedaling forwardly the additional small segments act to force the toothed wheel segments forwardly. In order to prevent "spraining" of the toothed wheel segments, they should be locked between discs. Such arrangement makes the known sprocket wheel complicated and also heavy, and upon penetration of dirt satisfactory operation becomes questionable.
Furthermore, owing to the presence of the additional small toothed wheel segments the maximally obtainable "speed" is rather limited because upon each rotation of the sprocket wheel larger angles will appear in the chain, which results in a less flexible drive and in irregular and greater wear. Moreover the chain will tend to rattle upon fast pedaling in the high position.
The present invention aims to improve the foregoing.